In a point-to-multipoint wireless communication system, a central station communicates throughout a coverage area with multiple remote stations. The remote station would be located somewhere within a node to communicate with a nodal transmitter receiver. A more complete description of such a system for example may be a point-to-multipoint system described by Langston application Ser. No. 08/345,183, filed Nov. 30, 1994, entitled from "Low Power, Short Range Point-To-Multipoint Communications System." Referring to FIG. 1 from that application, there is illustrated the distribution system network 10. This network comprises a plurality of nodes, such as node 11 through 13 covering a given combined area. For example, this area may cover a few city blocks only, but with hundreds of such nodes the area may comprise an entire large city. A central office 15 would be, for example, the central message center or source of programming for distribution to the other nodes 11 through 13. Each of the nodes would include a nodal broadcast transmitter or transmitter and receiver system. This is represented by system 11a, 12a, and 13a in the center of the nodes 11 through 13. The transmission from the central office 15 to the nodal broadcast transmitter systems 11a, 12a, or 13a may be done by cabling in or around the city such as fiber optic telephone exchange cables represented here as cable 16 between the central office 15 and the node transmitter systems 11a, 12a, and 13a. This coupling may also be done using microwave antennas such as using antenna 17 from the central office 15 communicating with antennas 18 at the center of the nodes at the systems 11a, 12a, or 13a. This distribution may be implemented in a variety of other configurations well known in the state of the art.
Referring to FIG. 2, there is illustrated a sketch of a nodal broadcast transmitter or system 12a. In the case of microwave distribution from a central office, there is the microwave antenna represented schematically by 18. The nodal transmitter broadcast systems 11a and 13a are like system 12a illustrated in FIG. 2, but with the polarizations indicated in the sketches as +45.degree. rotation and -45.degree. (315.degree.) rotation of FIG. 1. (Other orthogonal polarization may preferably be used, e.g., vertical and horizontal polarization). The system 12a includes a post 35 for supporting a four-quadrant sectorized antenna complex system represented by 12b. The system 12a includes a transmitter and receiver represented schematically by 21. Signals transmitted from system 12a are coupled from transmitter 21 to the nodal transmitter coverage or broadcast antenna system 12b comprised of four panel array antennas 31, 32, 33, and 34 via leads 22. The panel antennas 31-34 are mounted to post 35 via supports 22a housing transmission lines 22. Each of these panel antennas 31-34 comprises an array of transmitting antenna elements and receiving antenna elements as will be discussed in more detail later. Polarization of these antenna elements; 30 for antenna system 12b is such that panels 31 and 33 for system 12b transmit 45.degree. slant polarized waves (marked +45.degree. in FIG. 1), while panels 32 and 34 for system 12b transmit -45.degree. (315.degree.) slant polarized waves (marked -45.degree. slant in FIG. 1). See FIG. 1.
These panel antennas, for example, produce a 90 degree beam width so that each of the panel antennas 31-34 covers approximately 90 degrees beam width and the polarization from these panel antennas 31-34 alternates from +45.degree. slant polarization to -45.degree. slant (315.degree.) polarization to +45.degree. slant polarization and then to -45.degree. (315.degree.) polarization about the center of the node on post 35. This, therefore, provides a 360 degree pattern about the center of the node where the node broadcast transmitter is located.
The adjacent nodes 11 and 13 on either side of node 12 present orthogonal polarization. For example, the panel antennas 31 and 33 in nodes 11 and 13 produce -45.degree. (315.degree.) polarized signals and the panel antennas 32 and 34 in nodes 11 and 13 produce +45.degree. polarized signals. Therefore, at the adjacent sectors of the nodes, that is where node 11 is adjacent to node 12, the polarizations are orthogonal, and where node 12 meets node 13 the polarization is orthogonal. The node broadcast transmitting antennas systems 11b, 12b and 13b transmit and communicate with receiving stations 41 scattered in the area of the nodes.
Receiving station 41 may be located any-where in any of the nodes 11-13 and will receive the signal radiated from the antenna complex at the center of one of the nodes from the sectorized antenna system 20. The polarization of the receiving station 41 would determine which sector it is receiving from. For example, the receiving station 41a in FIG. 1 in node 12 would be in the +45.degree. slant polarized sector and receive +45.degree. slant polarized signals from the panel antenna 33 of system 12b. Station 41b in node 12 would receive preferably -45.degree. (315.degree.) slant polarized signals from the panel antenna 32 and not +45.degree. polarized signals from panels 31 or 33 from system 12b or from panel 34 of system 11b. For the receiving antenna 41c, located in the overlapping area of the pattern of 32 and 33, it is possible to receive both +45.degree. polarized signals from panel antenna 33 and -45.degree. (315.degree.) slant polarized signals from panel antenna 32. However, a signal received from the face which is of the wrong polarization from the antenna 41 would be 20 to 30 dB lower in power than the other face. For example, if the antenna of 41c was -45.degree. (315.degree.) polarized it would receive the signal from panel antenna (+45.degree. polarized) 33 of system 12b from 20 to 30 dB down from that of the -45.degree. (315.degree.) polarized signal from antenna 32 of system 12b. Similarly, stations 41e and 41f at the edges of nodes 11 and 12 and 12 and 13, respectively, achieve similar isolation based on polarization. That is, station 41e with a -45.degree. (315.degree.) polarized antenna picks up signals from panel antenna 32 of system 12b. The signals from panel antenna 34 of system 11b are 20 to 30 dB lower than those from panel antenna 32 of system 12b. Again the system may preferably be with the receiver antennas polarized vertical and/or horizontal to match the nodal transmitting antennas.
The system is designed such that the signals from all four panels 31-34 are transmitting at the same carrier frequencies but may contain different information. The system is dependent upon space (different coverage areas) as well as frequency and polarization diversity.
The system, as described herein, utilizes a four-quadrant sectorized antenna. However the system could be of any even number of sectors such as two, four, eight, etc. The four-quadrant sectorized antenna discussed in FIG. 1 has an azimuth pattern relatively flat over a plus and minus 45 degrees from the center (90.degree. beam width) which is easy to implement and is, therefore, more easily implemented, for example on a building or a tower for mounting panel antennas. An octagon, or eight sector, antenna system may be practical in some cases, but receivers located a line drawn to the corners will be able to see more than two panels and, hence, see signals having the same polarization. Thus, the four sector is the preferred embodiment where frequency re-use is required.
The transmission to and from the central office 15, as stated previously, may include a microwave network with the microwave dish 18 coupled to a transceiver via an up/down converter 18a. The received signals from the central office 15 are upconverted to millimeter waves and sent via the panel antennas and the received signals at the panel antennas are down converted to microwave frequencies and sent back to the central office 15 via antenna 18 or cables 16. (Other frequencies, e.g., millimeter frequency, may also be used to interconnect the nodes). This application is incorporated herein by reference.
Viewing a particular sector, for example, the base station BS or the node transmitter transmits over a given sector as represented within the dashed lines in FIG. 3. The subscriber or remote stations RS are represented by RS #1, RS #2, etc. in FIG. 3. The transmission from the node directional base station antennas BS in the system represents the down stream information and the signals from the remote stations RS represents the upstream signals in FIGS. 3-5.
At the high operating frequencies of the microwave systems, such as at the 27-30 GHz range, the accuracy of frequency setting devices becomes difficult to achieve. Microwave systems that use digital modulation must maintain good frequency tolerance. In a point-to-multipoint communication system, the remote station costs dominates the cost per subscriber. Remote stations need a low cost method for creating frequency reference.
For heterodyne receiver architectures, remote stations must have local oscillators with specific requirements on operating frequency and phase noise. For digitally modulated payload signals at microwave operating frequencies, the cost and complexity of local oscillators can add significantly to the cost of the remote stations. Normally, local oscillators create a signal on a specific frequency by using either a high-Q component such as a crystal or by synchronizing to a signal with a known frequency. High-Q component can be expensive, especially if oscillators use ovens to stabilize frequency drifts over the operating temperature range.
When multiple remote stations use closely spaced channels, errors in operating frequency can cause interference between the channels.
The signal strength at a remote station varies as a function of the distance between the base station and the remote station. Traditionally designers use automatic gain control (AGC) circuits to automatically adjust the gain of receivers and maximize the dynamic range of the payload signals. An AGC circuit requires a gain setting control signal that payload demodulators often create. Remote station wiring routes the control signal to a variable gain amplifier in the front end of the receiver and sets the gain at the receiver. However, in a physical configuration where the payload demodulator is remotely located from the front end of the receiver the control signal becomes expensive to route to the front end receives function. It is highly desirable to provide a system which avoids the wiring or telemetry for a control signal from the payload demodulator to the front end of the receiver.
Applicants' invention described herein overcomes the above problems.